1. Field of the Invention
This invention relates to electrical circuits for use in radio receivers fabricated as integrated circuits.
2. Description of Related Art
There has recently been proposed communication networks known as radio paging networks which will operate over a large area, eventually world-wide, and which transmit digital radio paging information to individual network subscribers on a time-slot basis on a subcarrier of local VHF stereo FM entertainment broadcast channels. With a pilot carrier of 19 KHz and stereo bands entending from 0-53 KHz, the subcarrier has a band width of 9.5 KHz centered at 66.5 KHz.
The subcarrier is received by a subscribers receiver, which may be incorporated in a wrist-watch as an integrated circuit. The type of receiver employed is a Barber receiver which has a principal advantage of high image rejection. The Barber receiver configuration as shown in FIG. 1 comprises an aerial 2, for receiving an incoming signal having a carrier frequency fc, coupled to an AGC controlled preamplifier 4. The incoming signal is then fed to two arms 6, 8 where it is mixed in mixers 10. 12 with signals, in phase quadrature with one another, from a local oscillator 14. The frequency of the local oscillator is identical with the carrier frequency fc. The mixed signals in the two arms are fed via coupling capacitors to low-pass filters 16, 18, and amplifiers 20. 22 to further mixers 24, 26 where the signals are mixed with signals, in phase quadrature with one another from an intermediate frequency oscillator 28. The signals in arms 6, 8, now raised in frequency by the i.sub.f frequency are summated at 30, limited in amplitude by a limiting amplifier 32, and then demodulated by a detector 34 to yield the digital paging signal.
The principal advantage of such a receiver is that of image rejection, since the carrier and local oscillator frequencies are identical and result in a spectrum folded about a base DC level. Further advantages are that the receiver is suitable for integration, having a low enternal component count, and on-chip filtering is possible, since the selectivity is defined at a low frequency.
A principal disadvantage is that the receiver is highly sensitive to phase or gain imbalances in the arms 6, 8 and it is therefore important that the signal provided by the local oscillator and intermediate frequency oscillator are accurately in phase quadrature, to within 1.degree.. Further it is important that the local oscillator and the intermediate frequency oscillator produce signals of accurately defined frequency and high spectral purity. Finally it is normally necessary to track the frequency of the intermediate frequency oscillator with the cut-off frequency of the low-pass filters in the arms, and it is necessary to maintain the phase quadrature relationship while the frequency is varied.
So far as concerns the problem of providing signals which are accurately in phase quadrature, there are many types of networks which will provide a 90.degree. phase shift, for example R-C phase shift networks or networks employing inductances. However problems raise when such networks are incorporated in an integrated circuit. With R-C networks there are large tolerances in the values of the integrated capacitors and it is therefore very difficult to preset the network and obtain an accurate phase shift. With inductive networks, inductances are difficult to fabricate in an integrated circuit and are difficult to preset in value.